The present invention relates to an active-matrix liquid crystal display device.
In liquid crystal display devices, displaying is performed by varying the optical properties of the liquid crystal layer disposed between the substrates by changing the alignment direction of the liquid crystal molecules in the layer by applying an electric field thereto.
The conventional active-matrix liquid crystal displays have predominantly been of the twisted nematic (TN) mode in which the direction of the electric field applied to the liquid crystal molecules is set to be substantially vertical to the substrate plane, and display is performed by making use of optical rotatory power of the liquid crystals.
On the other hand, a system which makes use of the birefringence effect of the liquid crystals by setting the direction of the electric field applied to the liquid crystals to be substantially parallel to the substrate plane by using interdigital electrodes (in-plane switching mode) has been proposed in, for instance, JP-B 63-21907 and WO 91/10936 (JP-T 5-505247). This in-plane switching mode has the advantages of wide viewing angle and low load capacity in comparison with the conventional TN mode, and is a promising technique for the development of active-matrix liquid crystal displays.
In this in-plane switching mode, however, since it utilizes the birefringence effect of the liquid crystals for making display, it is necessary to set the gap between the substrates (liquid crystal layer thickness) to be about 4 xcexcm, which is notably smaller than that of the TN mode (about 10 xcexcm), for obtaining a display performance equal to the TN mode. Generally, reduction of the layer thickness brings into relief the influence of display irregularity due to nonuniformity of the gap between the substrates, giving rise to such problems as deterioration of displayed image quality and reduction of yield resulting in lowered mass productivity.
The gap between the substrates is controlled to a specified value by dispersing the uniformly sized spherical polymer beads as spacer of the opposing substrates between which the liquid crystal layer is disposed.
In the active-matrix liquid crystal display devices, a level difference of up to about 1 xcexcm may be produced on the substrate surface at the active element forming section, and a certain degree of nonuniformity of the inter-substrate gap is inevitably produced at the pixel region, too, due to a delicate relation between said level difference and dispersion of said spacer beads.
In the in-plane switching mode, the same degree of gap irregularity represents a far greater rate of gap variation than in the TN mode because of smaller inter-substrate gap, so that the techniques for lessening or eliminating display irregularity due to nonuniformity of said gap are of vital importance for the in-plane switching mode.
Further, according to the known TN mode, there is no dependence of the threshold voltage on the gap between the substrates (due to voltage responsitivity), while according to the in-plane switching mode, since the gap between the substrates independently contributes to the threshold voltage (due to field responsitivity) together with the gap between electrodes (Oh-e, et al. Appl. Phys. Lett. 67 (26), 1996, pp 3895-3897), particularly severe control of the gap between the substrates is necessary.
The present invention is envisaged to solve the above problems, and for this purpose, it provides an active-matrix liquid crystal display device employing the in-plane switching mode, which is minimized in or almost free of nonuniformity or irregularity of display resulting from variation of the gap between the substrates, and which is also capable of displaying high-quality images and has excellent mass productivity.
The active-matrix liquid crystal display device according to the present invention comprises a group of electrodes for applying an electric field to the liquid crystal layer disposed between a pair of substrates, said electric field being parallel to the plane of said substrates, active elements provided in connection to said electrodes, and an alignment layer(s) which aligns the liquid crystal molecules in the substantially same direction at the interface between said liquid crystal layer and at least one of the opposing substrates, wherein the extrapolation length, which expresses the strength of torsional anchoring of the liquid crystal molecules and said alignment layer surface at one or both of the interfaces between said liquid crystal layer and said opposing substrates, is set to be not less than 10% of the gap between the substrates (liquid crystal layer thickness)
The xe2x80x9cextrapolation lengthxe2x80x9d refers to the increment of the apparent inter-substrate gap when the liquid crystal cell behaves like a cell having a greater inter-substrate gap than the actual gap in terms of the threshold characteristics on application of an electric field, in case the interfacial anchoring is weak and finite (de Gennes: The Physics of Liquid Crystal, Oxford University Press, 1974, page 75).
The alignment layer used in the present invention may be one in which the torsional anchoring coefficient A2 at the alignment layer surface against the liquid crystal molecules at the interface is less than 20 xcexcN/m.
Regarding the method for changing the optical properties according to the status of molecular alignment of said liquid crystal layer, it is expedient to use a pair of polarizers arranged to have their axes of polarization crossed at right angles with each other, and to select the parameter dxc2x7xcex94n (d: liquid crystal layer thickness; xcex94n: refractive index anisotropy of the liquid crystal composition) so as to satisfy the relation of 0.2 xcexcm less than dxc2x7xcex94n less than 0.5 xcexcm.
It is desirable that the controlled alignment direction of the liquid crystal molecules is substantially the same at the two interfaces between said liquid crystal layer and said pair of substrates.
Also, at least one of the alignment layers formed on said substrates is preferably made of an organic polymer containing a polymer and/or oligomer in which the polymerizate of long-chain alkylene groups and/or fluoro groups provided in the amine or acid moiety accounts for 5-30% of the total number of moles.
The polymer and/or oligomer used in said alignment layer are preferably those having a weight-average molecular weight of 2,000-90,000. The long-chain alkylene groups and/or fluoro groups in the polymer may be main chain type, side chain type or terminal type.
The alignment layer is preferably made of an organic polymer having long-chain alkylene groups and/or fluoro groups, which includes a polymer and/or oligomer-amic acid imide type, polymer and/or oligomer-imide type, polymer and/or oligomer-imidosiloxane type, and polymer and/or oligomer-amide-imide type. It is also possible to use an organic polymer obtained from dehydration ring-closing reaction of a polymer and/or oligomer-amic acid comprising a single-ring rigid diamine as amine moiety and an aliphatic tetracarboxylic acid dianhydride and/or an alicyclic tetracarboxylic acid dianhydride and an aromatic tetracarboxylic acid dianhydride having main chain type long-chain alkylene groups and/or fluoro groups as acid moiety.
According to an embodiment of active-matrix liquid crystal display device of the present invention, at least one of the alignment layers formed on the substrates may be an inorganic material layer. This inorganic material layer is preferably an inorganic alignment layer which has been surface treated by oblique evaporation technique. In case of using such an inorganic alignment layer, an organic alignment layer may be used as the other alignment layer. Such an organic alignment layer is preferably a layer of an organic polymer which has had a rubbing treatment.
According to an embodiment of the electrodes and active elements used in the present invention, it is desirable that these are formed only on one of the pairing substrates, and that the outermost surface of this substrate is constituted by an inorganic material layer.
According to another embodiment of active-matrix liquid crystal display device of the present invention, at least one of the alignment layers formed on the respective substrates may be a layer of a photoreactive material. Such a photoreactive material layer is preferably a photoreactive alignment layer which has been subjected to linearly polarized light irradiation treatment, and such a photoreactive alignment layer is preferably made of an organic polymer containing a polymer and/or oligomer having at least one diazobenzene group.